Calorimetric flow meter having high heat conductivity strips

ABSTRACT

A flow meter includes a heater for heating a fluid flow along a membrane. A temperature difference is measured between and upstream point and a downstream point. There are additionally provided one or more strips of material having a relatively high heat conductivity. Strips that are substantially perpendicular to the flow direction direct heat from the heater to the sides of the membrane, causing a large proportion of the heat that would otherwise drive heat flows to be dispersed and decreases inaccuracies or bias in the measured flow rate. Strips of material that are provided parallel to the flow direction act to direct heat from the heater along the direction of flow. This increases the proportion of heat that flows along this axis and guides the flow and hence reduces the proportion of heat available to drive heat flows that cause inaccuracies and bias in the measured flow rates.

The present invention relates to calorimetric flow meters and inparticular calorimetric flow meters implemented as integrated circuits.

Calorimetric flow meters measure mass flow and are used in industry andin the medical profession to measure the flow of a fluid. The fluidflows along a particular path defined by a suitable article and a heateris provided located halfway along the path for heating the middle of thepad, and the rest of the path being unheated. The fluid is heated by thepath. Sometimes several heaters are provided along the path. Examples ofarticles that may be suitable for defining a path include tubes andmembranes.

When the heater is on and no fluid flows the heater causes a temperaturedistribution along the path whereby the temperature drops with distancefrom the beater. When a fluid flow is present in the tube, heat exchangetakes place between the article defining the path and the fluid,Upstream from the heater fluid flow will cool the path defining articleslightly from the expected temperature distribution and downstream thefluid flow will cool the path defining article slightly is or even heatthe path defining article slightly relative to the zero flow temperaturedistribution. Temperature sensors positioned between the heater and theends of the path defining article are operative to detect the differenceΔT between the measured upstream and downstream temperatures.

The temperature difference ΔT between upstream and downstream isproportional to the mass flow rate φ_(m). Accordingly, this can be usedto calculate the mass flow rate. The relation between the temperaturedifference ΔT and mass flow rate φ_(m) is also dependant on the heatconductivity of the path defining article and the properties of thefluid such as pressure, humidity, viscosity and heat conductivity.

Typically, the temperature sensors and the heater are thermally isolatedto try to improve sensitivity. As a result of this thermal isolation themass flow rate φ_(m) to temperature difference ΔT relation shows muchgreater dependence on the properties of the fluid. This does notguarantee improved accuracy however as there are other heat flows withthe flow meter.

Firstly of all there is a conductive heat flow from the heater throughthe path defining article, which is typically minimized to increasesensitivity. Additionally, there is free convection within the fluid.This convection modifies the temperature distribution within the fluidflowing along the path (in inclined membranes, this effect is used asfor inclination sensing). The heat flow due to convection isparticularly dependent on the heater temperature, viscosity and heatconductivity of the fluid. There is also heat conduction through thefluid itself which is dependent upon its heat capacity, heatconductivity, pressure, temperature and humidity. Also Infraredradiation to or from the environment to the path can modify thetemperature distribution along the path.

Even if the heater and the temperature sensors are thermally isolated asmuch as is practicable from unwanted external heat flows, there arestill inaccuracies in the measured flow rates as a result of theadditional heat flows described above.

Additionally, it is known that variations in the temperature of thepathway can cause deviations from the linearity of the relation betweentemperature difference and flow rate. This is due to the variations inthe heat flows such as free convection, which are typically not linearlyproportional to temperature. This leads to inaccuracies in the measuredflow rate and particularly to offset drift.

It is therefore an object of the present invention to provide acalorimetric flow meter that alleviates or overcomes the above problems.

According to a first aspect of the present invention, there is provideda calorimetric flow meter comprising a pathway along which a fluid mayflow in a particular direction; a heating means for heating the fluidflow; and temperature sensing means provided at points upstream anddownstream of the heating means for measuring a temperature differencetherebetween wherein one or more strips of material having a relativelyhigh heat conductivity are provided adjacent to said heating means, saidstrip or strips having axes running in a substantially differentdirection to the direction of fluid flow.

The provision of the one or more strips of material having relativelyhigh heat conductivity adjacent to the heating means conducts heat awayfrom the heating means and reduces the proportion of the heat generatedby the heater available to power unwanted heat flows that causeinaccuracy or offset in the measured flow rate.

Preferably, the fluid flows along a pathway darned by a pathway definingarticle. The pathway defining article may be a membrane, a tube, a pipeor other suitable structure. The relatively high conductivity stripshave a relatively high heat conductivity compared with the pathway.

Preferably the axes of the strip or strips are substantiallyperpendicular to the direction of fluid flow. Typically two such stripsare provided, on either side of the fluid flow path.

The heating means may comprise a single heater or a plurality ofheaters. If the heating means comprises a plurality of heaters, they arepreferably dispersed evenly to the side of the flow path. In somepreferred embodiments, additional heating means are provided at both theupstream points and the downstream points.

In one preferred embodiment, there is provided a strip or strips ofmaterial with a relatively high heat conductivity running substantiallyparallel to and adjacent to the fluid flow path. This directs heat fromthe heater along the flow path and thus prevents the heat flowing tounwanted parts of the flow meter.

The temperature sensing means may comprise separate temperature sensorsat the upstream temperature measuring point and the downstreamtemperature measuring point, Alternatively, the temperature sensingmeans may comprise one or a plurality of thermopiles (or thermocouples)connected along the flow path between the upstream and downstreamtemperature measuring points. In some embodiments, the thermopile orthermopiles (or thermocouples) may comprise or be incorporated into oneor more of the strips of material with a relatively high heatconductivity ruining adjacent to and substantially parallel to the fluidflow path.

In sonic embodiments, the strip or the strips of material with arelatively high heat conductivity having axes substantiallyperpendicular to the direction of fluid flow may incorporate or compriseone or more thermopiles (or thermocouples).

The thermopiles lying parallel to the flow direction may be used todirectly read out a temperature difference between the upstream anddownstream points. To reduce the nonlinearity in the relation betweentemperature difference and flow, thermopiles perpendicular to the flowdirection may be monitored to see that a constant signal is maintainedbetween their ends adjacent to the heater and their other ends which maybe provided at a position that maintains a reference temperature or actsas a heat sink. This allows the heating means to be controlled such thatit is at a constant temperature and hence non-linearity in the flow ratetemperature difference relation is reduced.

According to a second aspect of the present invention, there is provideda calorimetric flow meter comprising a pathway along which a fluid mayflow in a particular direction; a heating means for heating the fluidflow; and temperature sensing means provided at points upstream anddownstream of the heating means for measuring a temperature differencetherebetween wherein one or more strips of material having a relativelyhigh heat conductivity are provided substantially parallel to andadjacent to the direction of fluid flow.

The flow meter of the second aspect of the present invention mayincorporate any or all features of the flow meter of the first aspect ofthe present invention as desired or as appropriate.

The provision of the one or more strips of material having relativelyhigh heat conductivity provided substantially parallel to and adjacentto the direction of fluid flow directs heat from the heater along theflow path and thus reduces the proportion of the heat generated by theheater available to power unwanted heat flows that cause inaccuracy oroffset in the measured flow rate.

In one particularly preferred embodiment of either the first or thesecond aspects of the present invention, the calorimetric flow metercomprises: a pathway along which a fluid may flow in a particulardirection; a heating means for heating the fluid flow; and temperaturesensing means provided at points upstream and downstream of the heatingmeans for measuring a temperature difference therebetween; one or morestrips of a material with a relatively high heat conductivity providedadjacent to said heating means, the axes of said strips runningsubstantially perpendicular to the particular direction of fluid flow;and one or more strips of a material with a relatively high heatconductivity provided adjacent to and substantially parallel to thefluid flow path. The temperature sensing means in such an embodiment maycomprise one or a plurality of thermopiles provided between saidupstream point and said downstream point, Said thermopile or thermopilesmay comprise or may be incorporated into strips of material withrelatively high heat conductivity provided adjacent to and substantiallyparallel to the fluid flow path.

According to a third aspect of the present invention, there is provideda method of determining the mass flow rate of a fluid in a calorimetricflow meter of the type having a pathway along which a fluid may flow ina particular direction; a heating means for heating the fluid flow; andtemperature sensing means provided at points upstream and downstream ofthe heating means for measuring a temperature difference therebetween;and a heater, temperature monitoring thermopile or thermopiles having arelatively high heat conductivity provided adjacent to said heater andhaving axes substantially perpendicular to the direction of fluid flowand operable to measure a temperature difference between the pathwayadjacent to the heater and a reference temperature comprising the stepsof controlling the power input to the heating means in response to theoutput of the heater monitoring thermopile or thermopiles so as tomaintain the output of said heater monitoring thermopile or thermopilessubstantially constant; measuring the temperature difference between theupstream and downstream points and thereby calculating the flow rate.

The method of the third aspect of the present invention may incorporateany or all features of the flow meters of the first aspect and/or secondaspects of the present invention or be used in conjunction with the flowmeters of the first aspect and/or second aspects of the presentinvention as desired or as appropriate.

Maintaining the temperature of the pathway as close to constant aspossible reduces temperature dependencies of the flow rate temperaturedifference relation much as possible. Accordingly, the measured flowrate is more accurate than with flow meters heated with a constantpower.

According to a fourth aspect of the present invention, there is provideda calorimetric flow meter comprising: a pathway along which a fluid mayflow in a particular direction; a primary heating means for heating thefluid flow; temperature sensing means provided at points upstream anddownstream of the primary heating means; upstream and downstream heatingmeans provided upstream and downstream of the primary heating means; anda control means operable to: monitor the fluid temperature at theupstream and downstream points; adjust the power supplied to each of theupstream and downstream heaters to maintain a substantially equaltemperature at the upstream and the downstream points; determine thedifference in power supplied to said upstream and downstream heaters;and thereby determine the mass flow rate of the fluid.

According to a fifth aspect of the present invention, there is provideda method of determining the mass flow rate of a fluid in a calorimetricflow meter comprising a pathway along which a fluid may flow in aparticular direction; a primary heating means for heating the fluidflow; temperature sensing means provided at points upstream anddownstream of the primary heating means; upstream and downstream heatingmeans provided upstream and downstream of the primary heating means; anda control means, the method comprising the steps of: monitoring thefluid temperature at the upstream and downstream points; adjusting thepower supplied to each of the upstream and downstream heaters tomaintain substantially a substantially equal temperature at the upstreamand the downstream points; determining the difference in power suppliedto said upstream and downstream heaters; and thereby determining themass flow rate of the fluid,

The flow meter of the fourth aspect of the present invention and themethod of the fifth aspect of the present invention may incorporate anyor all features of the flow meter of the first and second aspects of thepresent invention and/or any features of the method of the third aspectof the present invention as desired or as appropriate.

This provides an alternative method of determining the mass flow in acalorimetric flow meter. This type of flow meter also enables arelatively constant temperature to be maintained over as much of thefluid flow as possible improving accuracy and reducing offset drift. Insome embodiments, the ratio of the power supplied to the differentheaters may be used to estimate the mass flow rate.

The upstream and downstream heating means are preferably activated inalternative pulses when in use. The pulsing may be synchronised with ahigh speed clock. In such circumstances, each heating means may beactivated on its pulse only if heating is required. In such embodiments,the power difference between the heaters can be calculated by countingthe relative number of pulses upon which each heating means isactivated, The heating pulses of each heating means are pulse widthmodulated. The pulses may have a preset amplitude. In an alternativeembodiment, the amplitude of the pulses may be varied. In suchembodiments, the variation in amplitude may be used to determine thepower difference between the upstream and down stream heating means.

Preferably, the heating means are collectively controlled to maintainthe temperature adjacent to the primary heating means substantiallyconstant. This temperature regulation results in constant heat loss fromother means such as temperature dependent conduction even if the massflow rate varies. This therefore increases the accuracy of the flowmeter. In some embodiments the temperature adjacent to the primaryheating means may be maintained substantially equal to the temperatureadjacent to the upstream and downstream heating means.

Preferably, the primary heating means is used to supply most of the heatand the upstream and downstream heating means are used for compensatingthe heat conduction of the flowing fluid. Most preferably, either thetotal power input at the primary heating means or the total power inputat the upstream and downstream heating means combined may be keptconstant. This prevents temperature oscillations within the flow meter.The power ratio between the primary heating means and the upstream anddownstream heating means can be varied to increase the resolution of theflow meter and/or to optimise the linearity of the flow meter. Ofcourse, the total power of all heaters will be increased if the flowrate increases, otherwise a constant temperature above ambienttemperature cannot be maintained.

A further to sensing means, which may be a thermocouple or a thermopile,may be provided at the point where the fluid flow enters the passageway,This allows the input temperature of the fluid flow to be monitored andthe output of the heaters adjusted accordingly, if required.

The position of the upstream and downstream heating means may be eachoffset towards or away from the primary heating means relative to theupstream and downstream temperature sensing means. By placing theupstream and downstream heaters closer to the middle than the upstreamand downstream temperature sensing means, one can adjust the linearityof the sensor and adjust the modulation depth of the sensing function.

In order that the invention will be more clearly understood, somespecific embodiments will be described by way of example only and withreference to the attached drawings, in which:

FIG. 1 is a block diagram illustrating heat flows in a calorimetric flowmeter;

FIG. 2 is a block diagram illustrating a first embodiment of acalorimetric flow meter according to the present invention;

FIG. 3 is a block diagram illustrating a second embodiment of acalorimetric flow meter according to the present invention; and

FIG. 4 is a block diagram illustrating a further alternative of thesecond embodiment of the present invention.

Referring now to FIG. 1, a known calorimetric flow meter 100 comprises amembrane 101 along Which a flow of fluid 102 occurs. At the midpoint ofthe membrane 101 there is provided a heater 103. The heater 103 causes anon-constant temperature distribution over the membrane 101 as a resultof heat conduction through the membrane 101, the temperature beinghighest in the vicinity of the heater 103 and dropping as the separationfrom the heater 103 increases. If fluid is flowing along the membrane101 and is at a lower temperature than the membrane it will absorb heatfrom the membrane 101 and vary the temperature distribution, loweringthe temperature with respect to the expected temperature distributionupstream of the heater 103 and raising the temperature with respect tothe expected temperature distribution downstream of the heater 103. Thetemperature difference between upstream and downstream points T_(up),T_(down) each substantially equidistant from said heater 103 ismeasured. Due to this temperature difference heat will flow through themembrane opposite to the direction of the fluid flow. This isrepresented by the flow 114. Typically this measurement is made usingabsolute temperature sensing means at each point T_(up), T_(down). On asimple model, the temperature difference is proportional to mass flowand can thus be used to calculate the mass flow.

Deviations in the reliability of this model occur because there areother heat flows within the flow meter 100 than the heat flow 113associated with the fluid flow 102. For instance there is heat 112conducted, through the membrane 101 in directions other than thedirection of fluid flow 102. There is heat 111 conducted, radiated orconvected away from the surfaces of the membrane 101 and there is heat114 transferred by forced convection within the fluid, balanced with aflow through the membrane. This flow 114 makes the sensor work and if itis increased, reduces the additional heat flows 111 and 112. At highoperating temperatures also IR radiation is present. These additionalheat flows 111 and 112 cause inaccuracies and bias in flow ratemeasurements made using flow meters of the type 100.

Referring now to FIG. 2, a flow meter 200 according to a firstembodiment of the present invention is shown. As in the conventionalflow meter, heating means 203 is provided for heating a fluid flow 202along a membrane 201 and a temperature difference, ΔT, is measuredbetween and upstream point T_(up) and a downstream point T_(down.)

In this embodiment 200, there are additionally provided strips ofmaterial having a relatively high heat conductivity 204, 205. The stripor as shown in FIG. 2 pair of strips 204, substantially perpendicular tothe flow direction 202. direct heat from the heater 203 to the sides ofthe membrane 201. As these strip(s) 204 have a relatively large heatconductivity, a large proportion of the heat that would otherwise driveheat flows 111, 112 is dispersed and does not therefore causeinaccuracies or bias in the measured flow rate.

In the particular embodiment shown, a strip of material 205 having highheat conductivity relative to the membrane 201 is provided parallel tothe flow direction 202. This acts to direct heat flow from the heateralong the direction of flow. This increases the proportion of heat thatflows along this axis and guides the flow 114 and hence reduces theproportion of heat available to drive heat flows 111 and 112 which causeinaccuracies and bias in the measured flow rates.

Whilst in the embodiment of FIG. 2, both strips 204 and 205 are provide,the provision of strips 204 only or strips 205 only will provide someimprovement in performance over the known flow meter of FIG. 1.

In the embodiment of FIG. 2, the temperature difference ΔT betweenT_(up) and T_(down) may be measured using a thermocouple (or athermopile), This advantageously measures a temperature differencebetween T_(up) and T_(down) directly rather than indirectly. Thethermocouple (or thermopile) may he incorporated into the high heatconductivity ship 205. In preferred embodiments, the strip 205 is athermopile or a plurality of thermopiles. This reduces the complexity ofthe design.

Typically, the strips 204 may also incorporate a thermocouple orthermopiles such that the temperature of the heater 203 can be monitoredrelative to a reference temperature or heat sink provided at the edge ofthe membrane 201.

In embodiments wherein both strips 204 and 205 comprise thermopiles, theflow rate can be measured simply and accurately by reading the signalfrom thermopile 205. The accuracy of this measurement and inparticularly the linearity of the relation between the measuredtemperature difference and the flow rate can be maximized by maintainingthe heater 203 at a substantially constant temperature, this reduceschanges in the heat flows 111 and 112. The heater 203 can be maintainedat a constant temperature by using the signals from thermopiles 204 todetermine the variation in temperature between the heater 203 and thereference provided by the edge of the membrane 201 and adjust the powersupplied to the heater to maintain this temperature difference asconstant as possible.

Although this allows the temperature in the middle of the membrane 201to be kept constant, the temperature at T_(down) and T_(up) is stilldifferent. This can introduce some bias and inaccuracy but is animprovement over known flow meters such as that shown in FIG. 1.

Referring now to FIG. 3 a further alternative embodiment of a flow meter300 is shown. This embodiment is largely similar to that of FIG. 2having a membrane 301, a fluid flow 302, and thermopiles 304 and 305,however this embodiment comprises a primary heater 303 and twoadditional heaters 306, 307. The additional heaters 306, 307 comprise anupstream heater 306 located at T_(up) and a downstream heater 307located at T_(down). The additional heaters 306 and 307 can be used toreduce the temperature variation across the membrane 301 and hencereduce non-linear variations in the relation between ΔT and flow rate.

A first method of using such a sensor is to use only the additionalheaters 306, 307. These heaters 306, 307 are switched on alternativelyto deliver a pulse width modulated pulse of heat with a presetamplitude. The power difference in these heating elements replaces theheat flow 114. The switching may be synchronised with a high frequencyclock with a much smaller period than the thermal time constant of themembrane arrangement. The output of the thermopile 305 is monitored anda control loop is set up to maintain a substantially constanttemperature at T_(down) and T_(up). This is achieved by only switchingon each heater 306, 307 at the time for its heat pulse when it isnecessary to raise the temperature T_(down) or T_(up) to maintain theconstant temperature. By counting the number of clock pulses for whicheach heater is switched on, an input power difference can be determined(for no flow they will be both on for the same number of pulses). Fromthe input power difference the heat flow due to the mass can bedetermined and hence the mass flow can be calculated. The amplitude ofthe pulses may be varied to keep the temperature by heater 303 as stableas possible. Note that using a thermopile for the temperature monitoringhas the advantage that such a sensor does not show an offset. Since theoutput of this thermal sensor is regulated to zero, linearity and driftof the sensitivity do not affect the sensor functioning.

By keeping T_(down) and T_(up) at equal temperature the absolutetemperature of the middle and the ends of the flow will still exhibitsome non-linear variation with variation in the flow. This change canfarther be reduced by using the heater 303 to provide most of theconductive heat transferred to the fluid. This allows heaters 306, 307at T_(down) and T_(up) to be used merely to measure the flow rate.

In this method either the total power input at T_(down) and T_(up)should be kept constant or the power input by heater 303 should be keptconstant. If this is not the case, oscillations with the regulation ofthe middle heater will occur.

Using heater 303 to maintain the conductive heat, the temperaturedistribution on the membrane 301 is kept as constant as possible alongthe whole flow pathway. The control loop of this heater also minimisesthe effect of IR radiation. By varying the power ratio between theheater 303 and heaters 306 and 307, it is possible to set the dynamicrange of the flow meter or optimise the flow meter for linearity.

Referring now to FIG. 4, for additional accuracy, it is possible toprovide an additional temperature sensing means 308. At the pointwherein the fluid flow 302 enters the pathway. The output of thistemperature sensing means 308 may be used to measure the inputtemperature of the fluid 302 and may also be used in setting the powerinput to the heaters 303, 306 and 307 to accommodate variations in thetemperature of the incoming fluid or attempting to maintain as even aspossible a temperature along the full length of the pathway. Alsoprovided is temperature sensing means 309 for sensing the ambienttemperature. This diagram also shows connections allowing the variouscomponents to be controlled by a suitable control means.

It is of course to be understood that the invention is not to berestricted to the details of the above embodiments which are describedby way of example only. In particular whilst in the specific embodimentsabove the pathway defining article is a membrane, it is of coursepossible that an alternative suitable pathway defining article such as atube may be used instead.

1-7. (canceled)
 8. A calorimetric flow meter comprising: a pathway alongwhich a fluid may flow in a particular direction; a primary heatingmeans for heating the fluid flow; temperature sensing means provided atpoints upstream and downstream of the primary heating means; upstreamand downstream heating means provided upstream and downstream of theprimary heating means; and a control means comprising means operable to:monitor the fluid temperature at the upstream and downstream points;adjust the power supplied to each of the upstream and downstream heatersto maintain a substantially equal temperature at the upstream and thedownstream points; determine the difference in power supplied to saidupstream and downstream heaters; and thereby determine the mass flowrate of the fluid wherein the upstream and downstream heating means areactivated in alternate pulses.
 9. A calorimetric flow meter as claimedin claim 8 wherein temperature sensing means are provided adjacent tothe primary heating means.
 10. A calorimetric flow meter as claimed inclaim 8 wherein a further temperature sensing means, which may be athermocouple or a thermopile, is provided at the point where the fluidflow enters the passageway, for monitoring the input temperature of thefluid flow and the control means is operable to adjust the output of theheaters in response to the monitored input temperature of the fluidflow.
 11. A calorimetric flow meter as claimed in claim 8 wherein theposition of the upstream and downstream heating means are each offsettowards or away from the primary heating means relative to the upstreamand downstream temperature sensing means.
 12. A method of determiningthe mass flow rate of a fluid in a calorimetric flow meter comprising apathway along which a fluid may flow in a particular direction; aprimary heating means for heating the fluid flow; temperature sensingmeans provided at points upstream and downstream of the primary heatingmeans; upstream and downstream heating means provided upstream anddownstream of the primary heating means; and a control means, the methodcomprising the steps of: monitoring the fluid temperature at theupstream and downstream points; adjusting the power supplied to each ofthe upstream and downstream heaters to maintain substantially asubstantially equal temperature at the upstream and the downstreampoints; determining the difference in power supplied to said upstreamand downstream heaters; and thereby determining the mass flow rate ofthe fluid wherein the upstream and downstream heating means areactivated in alternate pulses.
 13. A method as claimed in claim 12wherein each heating means is activated on its pulse only if heating isrequired and the ratio of the power supplied to the different heaters iscalculated by counting the relative number of pulses upon which eachheating means is activated.
 14. A method as claimed in claim 12 whereinthe heating pulses of each heating means are pulse width modulated andthe heating pulses have a preset amplitude.
 15. A method as claimed inclaim 12 wherein the amplitude of the pulses is varied and the variationin heating pulse amplitude is used to determine the power differencebetween the upstream and downstream heating means.
 16. A method asclaimed in claim 12 wherein the temperature adjacent to the primaryheating means is monitored and the heating means are collectivelycontrolled to maintain the temperature adjacent to the primary heatingmeans substantially constant.
 17. A method as claimed in claim 16wherein the temperature adjacent to the primary heating means ismaintained substantially equal to the temperature adjacent to theupstream and downstream heating means.
 18. A method as claimed in claim12 wherein the primary heating means is used to supply most of the heatand the upstream and downstream heating means are used for compensatingthe heat conduction of the flowing fluid.
 19. A method as claimed inclaim 12 wherein either the total power input at the primary heatingmeans or the total power input at the upstream and downstream heatingmeans combined is kept constant.
 20. A method as claimed in claim 12wherein the input temperature of the fluid flow is monitored and theoutput of the heaters is adjusted in response to the monitored inputtemperature of the fluid flow.